1. Field of the Invention
This invention relates to a liquid delivery MOCVD process for deposition of high frequency dielectric material on a substrate. This invention also pertains in other aspects to capacitors utilizing a Ba--Sr--Ti-oxide dielectric material, to integrated circuit chips comprising such capacitors, and to a method of making such capacitors.
2. Description of the Related Art
High quality dielectric films possess numerous properties of technological importance, including dielectric constant, low loss of electrical characteristics, e.g., low electrical leakage character, and robustness in applications such as formation of capacitive structures for devices useful in wireless communications.
Examples of materials useful in forming such high quality dielectric films include barium titanates, barium-strontium titanates, and (Zr,Sn)TiO.sub.4.
In these dielectric material systems, precise and repeatable compositional control is required to produce reliable films of the desired quality. Physical deposition methods (e.g., sputtering, evaporation, etc.) to fabricate thin films are deficient in this respect, as are traditional approaches to metal-organic chemical vapor deposition (MOCVD) involving the use of bubblers. Chemical vapor deposition (CVD) is a particularly attractive method for forming high quality dielectric film layers because it is readily scaled up to production runs and because the electronic industry has a wide experience and an established equipment base in the use of CVD technology which can be applied in new CVD processes. In general, the control of key variables such as stoichiometry and film thickness, and the coating of a wide variety of substrate geometries is possible with CVD. Forming of thin films by CVD permits the integration of the film materials into existing device production technologies. CVD also permits the formation of layers of refractory materials that are epitaxially related to substrates having close crystal structures.
CVD requires however that the element source reagents, i.e., the precursor compounds and complexes containing the elements or components of interest, must be sufficiently volatile to permit gas phase transport into the chemical vapor deposition reactor. The elemental component source reagent must decompose in the CVD reactor to deposit only the desired element or components at the desired growth temperatures. Premature gas phase reactions leading to particulate formation must not occur, nor should the source reagent decompose in the lines before reaching the reactor deposition chamber. When compounds are desired to be deposited, obtaining optimal properties requires close control of stoichiometry which can best be achieved if the reagent can be delivered into the reactor in a controllable fashion. In this respect the reagents must not be so chemically stable that they are non-reactive in the deposition chamber.
Desirable CVD reagents therefore are fairly reactive and volatile. Unfortunately, for many of the high dielectric film materials described hereinabove, volatile reagents do not exist. Many potentially highly useful refractory materials have in common that one or more of their components are elements, i.e., the Group II metals barium, calcium, and strontium, as well as the early transition metals zirconium and hafnium, for which no or few volatile compounds well-suited for CVD are known. In many cases, the source reagents are solids whose sublimation temperatures may be very close to the decomposition temperature, in which case the reagent may begin to decompose in the lines before reaching the reactor, and it is consequently very difficult to control the stoichiometry of the deposited films from such decomposition-susceptible reagents.
In other cases, the CVD reagents may be liquids, but their delivery into the CVD reactor in the vapor phase is impractical due to problems of premature decomposition or stoichiometric control.
In multicomponent films such as high dielectric films, the constituent elements may form films with a wide range of stoichiometrics. In such film applications, the controlled delivery of known proportions of the source reagents into the CVD reactor chamber is essential to the achievement of suitable product films and microelectronic device structures.
While source reagent liquid delivery systems afford potential advantages over conventional techniques, there often is some fraction of the precursor composition that decomposes into very low volatility compounds that remain in the vaporization zone, when the precursor reagent is furnished in a liquid form.
In liquid delivery MOCVD, the precursor is dissolved or suspended in a liquid medium which is vaporized, and the resulting precursor vapor is passed to the chemical vapor deposition chamber. The problems attendant the use of such technique for film formation of high quality dielectric films includes the fact that virtually all solid and liquid precursors undergo some decomposition when heated for conversion to the gas phase, in the vaporization step, although this fraction is typically small for "well-behaved" compounds.
Additionally, CVD precursors often contain impurity species, and the presence of such species can cause undesirable thermally activated chemical reactions in the vaporization zone, also resulting in the formation of involatile solids and liquids at that location. Extraneous by-products may therefore be generated in the vaporization zone and pass to the deposition chamber, where they are undesirably incorporated into the growing dielectric film, with consequent adverse affect on the product film quality and character.
Despite the advantages of the liquid delivery approach, the foregoing deficiencies pose a serious impediment to the widespread use of the liquid delivery technique for providing volatilized reagents and forming high quality dielectric films.
Relating the foregoing to specific applications, it would be desirable if MOCVD could be efficiently utilized for the fabrication of capacitor structures employing high dielectric constant material, in applications such as communication circuits and other high-frequency circuits where discrete capacitors are typically used. It would also be desirable if thin film capacitors could be used instead of discrete ones, especially if the former could be integrated with the circuitry on an IC chip.
Among the (actual or potential) applications for integrated capacitors of the foregoing type are dynamic random access memory (DRAM) storage capacitors, feedthrough capacitors, bypass capacitors, capacitors for resistance-capacitance (RC) filters, and capacitors for switched capacitor filters. For DRAM storage capacitors, large specific capacitance and low leakage current are important requirements. For the other identified applications, large specific capacitance is also important, although it generally need not be as large as for DRAM applications. Among other important considerations for capacitors for non-DRAM applications are the achievement of low leakage current, high breakdown field, small loss tangent (or large quality factor), and small variation of capacitance with temperature. At least for those applications that require very low harmonic distortion or intermodulation distortion, low dependence of the capacitance on the applied voltage is very important.
It has recently been established that thin film capacitors with perovskite Ba.sub.1-x Sr.sub.x TiO.sub.3 dielectric can attain high specific capacitance characteristics. However, such capacitors have relatively large voltage dependence of the dielectric constant k that makes them unsuitable for some important applications.
Recently it was also discovered that capacitors with a non-perovskite dielectric of the composition Ba.sub.2 Ti.sub.9 O.sub.20 could be fabricated, having a low dependence of capacitance on the applied voltage. There are several closely related phases of Ba--Ti--O containing material with similar properties, and any ceramic or thin film is likely to contain a mixture of these phases, as well as (in the case of a thin film) possibly amorphous components. Therefore, in the ensuing description, this material is referred to as Ba.sub.2 Ti.sub.9 O.sub.20, with the understanding that it may be a more complex mixture of phases. In the case where Sr is added, the designation (Ba,Sr)Ti.sub.9 O.sub.20 will be used, again with the understanding that composition will vary, as described hereinafter, over a wider range of stoichiometries than that indicated by the subscripts used.
Typically, the quadratic coefficient a.sub.2 for such non-perovskite materials is less than 100 ppm/V.sup.2. The coefficients a.sub.1 and a.sub.2 are defined by the expression .DELTA.C/C=a.sub.1 V+a.sub.2 V.sup.2, wherein C is capacitance, .DELTA.C is the change in capacitance, and V is applied voltage. The linear term a.sub.1 can be cancelled by means of a differential circuit, but the quadratic term a.sub.2 has to be minimized by appropriate choice of material of construction. Capacitors with Ba.sub.2 Ti.sub.9 O.sub.20 dielectric have relatively low specific capacitance, typically only about 5% of that of the perovskite Ba.sub.1-x Sr.sub.x TiO.sub.3.
It clearly would be desirable to have available integrated capacitors with a low voltage dependence characteristic for k, comparable to that of non-perovskite Ba.sub.2 Ti.sub.9 O.sub.20 (i.e., a.sub.2 &lt;100 ppm.sub.V.sup.2), and with higher k (and thus with higher specific capacitance) than is attainable with Ba.sub.2 Ti.sub.9 O.sub.20 (e.g., k greater than 40).
Accordingly, it is an object of the present invention to provide an improved liquid delivery chemical vapor deposition process for the formation of high quality dielectric films on substrates.
It is another object of the present invention to provide integrated capacitors comprising high quality dielectric films of such type, for use in applications such as switched capacitor filters (SCF) and other applications requiring capacitors with relatively high specific capacitance and low voltage dependence of the dielectric constant.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.